The hardware required to view such broadcasts is not excessive, but in the high-ish end, which lowers the audience a bit - the biggest problem is, as has been mentioned, the bandwidth. Viewers would need a broadband connection, with no congestion upstream. This generally limits you to more technically minded people, who usually like their info in easily-browsable text.
For the broadcasters though, the problem is also bandwidth. It’s not cheap, as anyone who’se even looked into hosting knows, and as web stuff goes, video broadcasting is the most expensive, bitwise. I could run streaming video off my webcam and Linux server myself right now, but if any more than a 1/2 dozen or so of my closest friends wanted a peek, I’d be paying Shaw extra $$$ pretty fast. Compare that with shelling out some dough initially (2000$? I have no idea) for a small transmitter. Anyone in reception distance can watch with no extra cost to me.
On top of all that, there are other problems like the quality being nowhere near that of TV, advertising oportunities being lower than that of a simple webpage, and the amazing competion of everyone else on the 'net. About the only places that work ok with streaming media are the NASA site when a mission is doing something exciting (they’re government funded though, and even that got crushed when Pathfinder landed), and pr0n sites (who get their $$$ up front and can afford everything because of it).
I guess there’s the problem. You could probably get streaming TV channels if lots of people were willing to pay for them, but since anyone with a 50$ TV and rabbit ears can get almost anything, there’s just no demand.